The race between two drug makers to market a generic version of a blockbuster blood thinner has grown so intense that one company hired the international detective and security firm Kroll, prompting an inquiry by two key senators.

Amphastar Pharmaceuticals Inc. and Momenta Pharmaceuticals Inc. have hired high-profile Washington lobbyists. Both companies are seeking approval for generic versions of low-molecular-weight heparin, which prevents blood clots. The only version of the drug available is Sanofi-Aventis SA's Lovenox, which lost patent protection about a year ago; it had global sales of $3.5 billion in 2008.

In 2003, Amphastar, of Rancho Cucamonga, Calif., applied to the Food and Drug Administration to make generic Lovenox. Momenta, based in Cambridge, Mass., applied two years later.

In 2007, the FDA told Amphastar that its prototype matched Lovenox chemically, a necessary condition for marketing. But, at the same time, the FDA unexpectedly set additional safety requirements for heparin. Both companies have submitted more data on their products and are waiting to see if they meet FDA standards.

This summer, Amphastar hired Tony Podesta, a Democratic power broker with ties to the White House who heads a major lobbying firm. It also added Republican lobbyist Mary Arnold and former federal prosecutor Michael Volkov. The company has spent about $700,000 on other lobbyists since 2006.

Amphastar's general counsel, Jason Shandell, said the company hired the three lobbyists "to help us enhance our communication with [Capitol] Hill," adding, "We like our new lobbyist team."

Since 2007, Momenta has spent about $700,000 on strategists, including Republican lobbyist Mark Isakowitz and Democratic lobbyist Chris Jennings, who worked in the Clinton administration.

Momenta said it hired lobbyists to promote legislation that would allow the FDA to approve generic biologic drugs, which are derived from proteins. Generic biologics aren't approved in the U.S. Congress, biotech companies and others are debating what standards the FDA should use for approving them.

Earlier this year, Amphastar sent the FDA a formal complaint against drug regulator Janet Woodcock, saying she had a conflict of interest with regard to heparin due to an alleged relationship with Momenta. The FDA and Department of Health and Human Services' inspector general are investigating.

Amphastar has noted, among other things, that Dr. Woodcock co-wrote a scientific paper with Momenta scientists in 2008.

Through an FDA spokeswoman, Dr. Woodcock said she couldn't comment. Momenta said there was nothing improper in its relationship with the regulator.

Concerned Momenta was making inroads at the FDA, Amphastar this summer hired Kroll, a Marsh & McLennan Cos. unit that does financial investigations, to look into Amphastar's problems and Dr. Woodcock's background. That prompted a Sept. 24 letter from the Senate Finance Committee's chairman, Max Baucus (D., Mont.), and ranking Republican, Chuck Grassley of Iowa, in which the two senators said it was "troubling" Amphastar was hiring an outside firm to investigate one of its regulators. The senators asked Amphastar for documents on the matter.

Amphastar's Mr. Shandell said his company did nothing improper and was justified in seeking an explanation for "years of abnormal roadblocks and moving targets" at the FDA. He said information on the Web suggested a conflict of interest between Dr. Woodcock and a co-founder of Momenta, which led Amphastar "to retain the services of Kroll to verify our public findings." He said the information Kroll provided on Dr. Woodcock was publicly available, and that Amphastar would cooperate with the Finance Committee inquiry.